As shown in FIG. 1, the light emitted from the lamp tube 1 is to be focussed by the lens 2 and then transmitted to the photoelectric converting module 3 for converting the optical signal of the focussed image into the electronic signal. Since the emitted light, through lens 2, has an incident angle 0 upon photoelectric module 13, the optical strength must be operated with a factor of cos.sup.4 .theta. so that the light strength received at two ends of photoelectric converting module 13 will be weaker than that at the intermediate portion of photoelectric converting module 13. The situation that photoelectric converting module 13 receives an unequal light strength will be made worse owing to the fact that tube 1 has originally an intermediate portion having a light strength larger than that at two ends thereof.
As shown in FIG. 2, ideally, if a photoelectric converting module 4 receives an equal light strength, the pixel output will present a straight line 5. Through the cos.sup.4 .theta. factor and the intermediate portion of the lamp tube having a stronger light strength, as shown in FIG. 3, the saturation (or resolution or light strength) in the middle portion of the photoelectric converting module 6 corresponding to the intermediate portion of the lamp tube will be about 50% higher (stronger) than that at two ends of photoelectric converting module 6 to present a convex pixel output line 7.
In order to overcome the above situation, several solutions are proposed. As shown in FIG. 4, a reflection piece is normally used to enhance the output of the photoelectric converting module. In order to equalize the light strength received by the photoelectric converting module as shown in FIG. 5, different reflection pieces are required around the intermediate portion and two ends of the lamp tube. Since the different reflection pieces must be arranged around the lamp tube in a precise location and a proper orientation, they cannot be easily mounted in position accurately.
As shown in FIG. 6, if we attempt to use a mask or a lamp cover for evening the light strength or lowering the light strength from the intermediate portion of the lamp tube, the light strength at two ends of the lamp tube will also be reduced.
As shown in FIG. 7, if we try to increase the lightness of the lamp tube by raising the lamp current, the lamp life will be shortened, more energy will be consumed, and there will come the problem that the vaporized mercury will reach a critical state which will intermittently spark which means we have an unstable light source.
In some hand-held scanner, the flat column lens is used which can increase the lightness but no lightness-equalizing effect is available as shown in FIG. 8.
If an L- or U-lamp tube is used for reinforcing the lightness in two ends of the lamp tube for trying to equalize the received light strength, the light strength output line still is not straight as desired as can be seen in FIG. 9.
It is therefore tried by the Applicant to deal with the above situation encountered in the prior art.